One of the grdcut bugs

The bug shown in
```
grdcut @earth_relief_03s_g -R-10/-9.8/4.9/5.0 -Glixo.grd
```

Was introduced in the commit

4/7/2023
SHA-1: bf4918172f8f627963180898334740887e67f42a

* allow slop in wesn (#7356)

But note that that commit only made appear the peak at the SW corner, the one at the NW is cause by a much older code (don't know more).